Very few people leave reviews, from my experience and reading about other artist's experiences. When I make purchases, I leave favourable reviews when I think the model deserves it. When I download something and it doesn't meet my expectations, because either I didn't read through the description, or didn't ask the artist about the scale (or whatever) I don't leave a review.
I think it was mentioned, that an absence of a review indicated that the model was adequate for their needs. Repeat business from the same customer also means that your work is valued.
There also have been discussions on this forum, that the metrics used to measure artists are unfair. I agree that those metrics need to be revised.
There are those people who don't read model descriptions, download something, and expect you to modify your model for free to suit their needs. When you refuse, they leave you an unfavourable review. I'm unsure if CGtrader support would remove those, if the artist thinks they are unfair.